Low-voltage differential signaling speeds data transfers
Today`s high-speed high-resolution cameras are demanding data-transmission methods that can carry data at speeds greater than 50 Mbits/s per wire pair to computer-based systems located 10 to 15 m away. Moving imaging data from camera to frame grabber requires a platform that consumes minimum power, generates little internal noise, resists external noise, and entails low cost.
One such platform, the low-voltage differential signaling (LVDS) standard, is being rapidly adopted by frame-grabber and camera manufacturers. As a low-voltage swing, differential-signaling technology, LVDS allows data transmission at hundreds of megabits per second. Better still, its low-swing and current-mode driver outputs create low noise and consume little power across the frequency spectrum.
"The differential data-transmission method used in LVDS is less susceptible to common-mode noise than single-ended schemes," says Patrick Brockett, vice president and general manager of National Semiconductor`s Analog Group (Santa Clara, CA). Differential transmission uses two wires with opposite current/voltage swings instead of the one wire used in single-ended methods to convey data. "The advantage of the differential approach is that noise is coupled onto the two wires as common mode and is thus rejected by receivers that look at only the difference between the two signals," Brockett says.
At present, the LVDS standard has been adopted by camera-vendor Dalsa (Waterloo, Ontario, Canada) and frame-grabber manufacturer Matrox (Montreal, Quebec, Canada). The line drivers and the receivers for Dalsa`s CA-D6 955-frame/s, 532 ¥ 516 progressive-scan camera and Matrox`s Genesis PCI-based frame grabber are supplied by National Semiconductor (NS).
The Dalsa CA-D6 camera uses four, parallel 8-bit data streams at 25 MHz to produce a 100-MHz-output rate. "By using the NS DS90C031 quad-CMOS differential line drivers," says Patrick Myles of Dalsa, "the CA-D6 camera employs two drivers per 8-bit data stream, or eight drivers for four data streams. The frame grabber is then responsible for re-integrating the data streams," he adds.
Typical of such designs is the optional LVDS digital input module that Matrox provides for its Genesis frame-grabber board. Providing up to 32-bit-wide LVDS inputs at up to 40 MHz, the add-in board also contains the NS DS90C032 line receiver and DS90C031 line driver. According to Matrox, these components were chosen after talking to several camera manufacturers.
The advantages of LVDS technology are not restricted to cameras and frame grabbers. Last month, National Semiconductor released a family of communication interface ICs that extends the advantages of LVDS technology to bus/backplane systems. For example, the Bus-LVDS IC family provides speeds of 155 to 400 Mbit/s, low power, and low noise/EMI. According to Brockett, the initial family consists of five devices: a low-skew nine-channel transceiver, a 10:1 serializer and a 1:10 deserializer chip set, a bus transceiver, and a 1:2 bus repeater.
In high-bandwidth communications applications, the Bus-LVDS devices allow system designers to increase bandwidth rates. "Bus-LVDS devices can drive a 20-card backplane at 155 Mbits/s while allowing important bus features such as high impedance on power-down and hot-insertion capabilities," says Brockett.
